1. Field of the invention
The present invention relates to a thermostable amylase, a process for the production thereof, a microorganism producing amylase, and a process for the production of maltooligosaccharides using the amylase.
2. Description of the Related Art
Oligo saccarides derived from starch, such as maltose, maltotriose, and the like, are used in various fields; most widely in the food industry, due to their superior properties as a base material for food production and a sweetening agent. Further, the demand for purified maltose as an ingredient of transfusion liquids has also increased, due to the superior properties thereof when compared with the conventionally used glucose. There is also an increasing demand for an oligosaccharide mixture containing maltotriose, obtained as a byproduct of the production of a purified maltose, having a mild sweetness and a viscosity lower than conventionally used syrups mainly comprising dextrin.
In a conventional process for the production of starch-hydrolyzation products, for example, oligosaccharides such as maltose, maltotriose and the like, raw starch is suspended in water at a high density, the starch suspension is heated to a temperature of between 85.degree. C. and 120.degree. C. to gelatinyze the suspension and form a starch paste having a high viscosity, the starch paste is treated with a starch-liquefying enzyme such as .alpha.-amylase, to lower the viscosity thereof by a partial cleavage of the starch chain, and the liquefied starch solution is treated with a saccharifying enzyme such as glucoamylase, .beta.-amylase or the like, depending on a desired product. In this conventional process, the gelatinyzing process and liquefying process require a large amount of energy. Moreover, due to the high viscosity of the starch paste, the gelatinyzing process and liquefying process require special apparatuses, and thus the cost of production of the final product is high. Furthermore, the liquefaction of the starch paste at a high temperature stimulates the aging of the liquefied starch and provides an isomerization of reducing terminal glucose residues of the liquefied starch, both of which result in a decreased yield of a final product.
To solve these problems, attempts have been made to directly treat raw starch with an enzyme having the ability to hydrolyze raw starch, and omitting the gelatinyzing and liquefying steps. For example, a method has been proposed in which a non-steaming alcohol fermentation process has been attempted wherein a saccharifying enzyme having a strong ability to hydrolyze raw starch is used. But this process is not practically useful.
Regarding enzymes which hydrolyze raw starch, Japanese Unexamined Patent Publication (Kokai) Nos. 60-83595 and 61-83595 disclose a raw starch-hydrolyzing glucoamylase produced by a fungus belonging to the genus Aspergillus; Japanese Unexamined Patent Publications (Kokai) Nos. 63-59881 and 63-59887 disclose a strong raw starch-hydrolyzing ability in the presence of .alpha.-amylase, provided by a raw starch-hydrolyzing enzyme produced by a fungus belonging to the genus Penicillum: and Japanese Unexamined Patent Publication (Kokai) No. 61-47189 discloses a raw starch-hydrolyzing glucoamylase produced by a fungus belonging to the genus Humicola.
In the saccharification of raw starch using a raw starch hydrolizing enzyme, taking into consideration the saccharification rate and a prevention of contamination by microorganisms, the enzyme should act on the raw starch at a temperature lower than a temperature at which raw starch is gelatinyzed, but higher than a temperature at which contaminating microorganisms can grow. More specifically, the raw starch hydrolyzing enzyme should act at 45.degree. C. to 70.degree. C., more preferably 55.degree. C. to 65.degree. C. But .beta.-amylases which act on raw starch and are stable at such temperatures are not known.
As enzymes which hydrolyze starch to provide maltose, .beta.-amylase of plant origin, for example, barley .beta.-amylase, and soybean .beta.-amylase, are conventionally used, but in addition to the above problems of thermostability, .beta.-amylases of plant origin also pose the problem of a stable supply thereof in a sufficient amount. Therefore, a .beta.-amylase of microbial origin, which is thermostable and is readily available in amounts sufficient for industrial use, is urgently needed, but .beta.-amylases of microbial origin which meet the requirements of industry are not known, although various kinds of .beta.-amylase of microbial origin have been reported.
A common disadvantage among conventional .beta.-amylases is that, since .beta.-amylases cannot hydrolize the .alpha.-1,6 glycoside bond in starch, a debranching enzyme such as isoamylase, pullulanase, or the like must be used in combination with the .beta.-amylase to provide a satisfactory yield. These debranching enzymes, however, are expensive.